(1) Field of the Invention
The present invention relates in general to an abrasion-resistant coating composition, as well as to a process for making such a composition and articles, in particular optical articles such as ophtalmic lenses, comprising an abrasion-resistant coating including a cured layer of said abrasion-resistant coating composition.
(2) Description of the Prior Art
Numerous articles must be provided with abrasion-resistant or scratch resistant coatings since their sensitivity towards scratching does not allows them to be used in practice or allows only short periods of use.
This is in particular the case of optical articles such as ophtalmic lenses for which an abrasion-resistant or scratch-resistant surface is essential for maintaining a lens transparency over a period of time as long as possible.
The use of transparent plastic materials for making optical articles such as ophthalmic lenses is nowadays common pratice due to their unique lightness advantage compared to mineral glasses. Additionally, the use of such transparent plastic materials for making optical articles, in particular ophthalmic lenses, has allowed manufacturing articles having high refractive indices of 1.6 or more.
As a result of this achievement, it has been possible to manufacture lenses of lower thickness for an equivalent corrective power (optical power).
However, plastic materials in general, and more especially those employed in the manufacture of ophthalmic lenses, usually have poor mechanical surface characteristics with in particular low resistance to abrasion or scratching.
It is therefore a common practice to protect them with an abrasion-resistant or scratch-resistant coating.
Aqueous composition of organoalkoxisilanes and metal alkoxides have been proposed for making such abrasion and scratch-resistant coatings, as for example in U.S. Pat. Nos. 4,084,021; 4,746,366; 4,754,012; 4,814,017 and 5,357,024.
Although these prior art compositions impart abrasion and scratch resistance to plastic substrates coated therewith there is still a need, in particular in the field of ophthalmic lenses, to formulate abrasion and scratch-resistant coating compositions exhibiting higher abrasion and scratch-resistance properties.
Furthermore, with the present possibility of obtaining transparent plastic material substrates having a wide range of refractive indices, for example, from 1.45 to 1.65 and more, it would be of major interest to formulate abrasion and scratch-resistant coating compositions which would exhibit refractive indices matching such a wide range of refractive indices and in particular high refractive indices.
Increasing the amount of metal alkoxides, such as titanium and zirconium alkoxide in the aqueous compositions of organoalkoxysilanes of prior art would increase the refractive indices of resulting cured coatings. However, high contents of metal alkoxides in such coating compositions lead to stability problems of the formulated compositions. In particular, the metal alkoxides, when used in relatively high amounts in the coating compositions, have a pronounced tendency to precipitate and agglomerate, resulting in an non-homogeneous coating.